This invention relates to compression/expansion refrigeration, and is particularly concerned with air conditioning systems wherein a sub-cooler is employed to reduce the relative humidity, that is, to increase the amount of latent cooling in the air leaving the indoor air evaporator.
Single-fluid two-phase air conditioning and refrigeration systems typically employ a compressor that receives the two-phase working fluid as a low temperature, low-pressure vapor and discharges it as a high temperature, high-pressure vapor. The working fluid is then passed to an outdoor condenser coil or heat exchanger, where the heat of compression is discharged from the working fluid to the outside air, condensing the working fluid from vapor to liquid. This high-pressure liquid is then supplied through an expansion device, e.g., a fixed or adjustable expansion valve or a pressure-reducing orifice, and then enters an indoor evaporator coil at low pressure. At this stage, the working fluid is a bi-phase fluid (containing both liquid and vapor phases), and absorbs heat from the indoor, comfort-zone air, so that the liquid phase is converted to vapor. This completes the cycle, and the vapor returns to the suction side of the compressor.
When warm indoor air passes through the evaporator coil, its temperature is lowered as it loses heat to the cold evaporator coil. As the air temperature is reduced to or below the dewpoint, moisture condenses on the evaporator coil and is removed from the indoor air. The actual temperature of the leaving air is reduced (i.e., sensible cooling), and the air is also dehumidified (i.e., latent cooling). The amount of latent cooling, or dehumidification, depends on whether the moisture in the indoor air will leave the air and condense on the evaporator coil.
Condensation of water vapor in the indoor air will take place only if the evaporator coil temperature is below the dewpoint of the air passing through, dewpoint being understood to be the temperature at which the water condenses in air.
Current standards on indoor air quality stress the need for controlled humidity in occupied spaces. High humidity has been identified as a major contributory factor in the growth of pathogenic or allergenic organisms. Preferably, the relative humidity in an occupied space should be maintained at 30% to 60%. In addition to adverse effects on human comfort and human health, high humidity can contribute to poor product quality in many manufacturing processes, and can render many refrigeration systems inefficient, such as open freezers in supermarkets. Also high humidity can destroy valuable works of art, library books, or archival documents.
In very, warm, humid conditions, a conventional air conditioner as just described can use up most of its cooling capacity to cool the air to the dewpoint (sensible cooling), and will have little remaining capacity for dehumidification (latent cooling).
The conventional approach to this problem of removing large mounts of humidity in a hot, humid environment has been to operate the air conditioner longer, by lowering the thermostat setpoint and over-cooling the air. This of course means that the air conditioner has to operate longer and will consume more energy. In addition, this practice results in blowing uncomfortably cold air onto persons in the indoor comfort space. In essence, overcooling lowers the temperature of the evaporator coil to allow more condensation on the coil. However, this makes the supply air too cold for human comfort. In order to restore the indoor air to a comfortable temperature, it is sometimes the practice to reheat the leaving supply air before it is returned to the comfort space. The indoor air temperature is raised to a comfortable level using either a heating element or a coil carrying the hot compressed vapor from the compressor, to raise the temperature (and reduce the relative humidity) of the overcooled air. In the case of either the heating element or the hot vapor coil, more energy is required.
One recent proposal for increasing the latent cooling of an air conditioning system, at low energy cost, has been a heat pipe. A heat pipe is a simple, passive arrangement of interconnected heat exchanger coils that contain a heat transfer agent (usually a refrigerant such as R-22). A heat pipe system can increase the dehumidification capacity of an air conditioning system, and reduce the energy consumption relative to the overcooling/reheating practice described just above. The heat pipe system is attractive because it can transfer heat from one point to another without the need for energy input. One heat exchanger of the heat pipe is placed in the warm air entering the evaporator, and the other heat exchanger is placed in the cold air leaving the evaporator. The entering air warms the refrigerant in the entering side heat exchanger of the heat pipe system, and the refrigerant vapor moves to the leaving side heat exchanger, where it transfers its heat to the leaving air and condenses. Then the condensed refrigerant recirculates, by gravity or capillary action, back to the entering side heat exchanger, and the cycle continues.
The heat pipe system built into an air conditioner can increase the amount of latent cooling while maintaining the sensible cooling at the preferred comfortable thermostat setpoint. In circumstances where the need for moisture removal is high, or where it is critical to keep the relative humidity below some point, the standard air conditioning system may not be able to deal effectively with high temperature and high humidity cooling loads. However, a heat-pipe enhanced air conditioning system cools the entering air before it reaches the air conditioner's evaporator coil. The entering side heat pipe heat exchanger pre-cools the entering air, so that less sensible cooling is required for the evaporator coil, leaving a greater capacity for latent cooling or dehumidification. The indoor supply air leaving the evaporator, being colder than the desired temperature, condenses the vapor in the leaving side heat pipe heat exchanger, which brings the supply air temperature back to the desired comfort temperature.
While the heat pipe arrangement does have certain advantages, such as passivity and simplicity, it has disadvantages as well. For example, the heat pipe is always in circuit, and cannot be simply turned off, even when increased sensible cooling without dehumidification is called for. In addition, because there are two heat-pipe heat exchanger coils in the indoor air path in addition to the evaporator coil, the indoor air flow can be significantly restricted. Also, it can be difficult to retrofit an existing air conditioner to accommodate the two additional coils in the same cabinet as the evaporator, and quite often a considerable amount of equipment has to be repositioned, and the cabinet enlarged, to accommodate the heat pipe.